The major export product from photosynthesis is glyceraldehyde-3-phosphate (G3P), a triose phosphate carbohydrate, which can enter either the starch or sucrose biosynthesis pathway depending on conditions in the cell. During the daytime, much of the carbon that is fixed by photosynthesis remains in the chloroplast and enters the starch biosynthesis pathway. At night, carbon stored in the form of starch is mobilized by conversion to sucrose, which is synthesized in the cytoplasm.
Starch, formally known as α-amylose, is a long-chain polysaccharide made of α 1→4 linked glucose, where the chain length numbers in the hundreds or thousands. α-amylose forms a single helix structure because of its regular repeating pattern, and this secondary structure readily crystallizes. The first step in the synthesis of α-amylose is the formation of hexose phosphates, including fructose 6-phosphate, glucose 6-phosphate, and glucose 1-phosphate. Glucose 1-phosphate is further ‘activated’ by reacting with the sugar nucleoside ATP to produce ADP-glucose. This form of glucose is highly reactive and readily joins an elongating chain of α-amylose at the 4-carbon position to give the characteristic α 1→4 linkage.
While α-amylose represents about 30% of the total starch in most plants, the rest of the starch is in a highly branched form called amylopectin. Rather than forming straight chain helices that readily crystallize, amylopectin does not crystallize. Amylopectins are formed by starch branching enzymes that form branches among short α-amylose chains that are α 1→6 glycosidic bonds.
When triose phosphates are exported from the chloroplast, they enter the sucrose biosynthetic pathway in a similar manner as the start of the starch pathway — by condensation to form a pool of hexose phosphates. Also like starch biosynthesis, glucose 1-phosphate reacts with a sugar nucleoside, in this case UTP instead of ATP, to form UDP-glucose. Sucrose is the result of the condensation reaction between this UDP-glucose and fructose 6-phosphate. This sucrose serves as the major form of transportable carbohydrate within the plant.
What determines whether the triose phosphates formed by photosynthesis enter the starch or sucrose pathways is the activity of a chloroplast envelope transporter. This transporter, called the triose phosphate-Pi antiporter, exchanges triose phosphates for Pi (inorganic phosphate) between the stroma and cytoplasm. When concentrations of Pi are high in the cyctoplasm, the antiporter is activated and exports triose phosphates in exchange for the uptake of Pi, with the cytoplasmic triose phosphate entering sucrose synthesis. On the other hand, when cytoplasmic Pi is low, no exchange happens and triose phosphates remain in the chloroplast to enter the starch synthesis pathway.